I would second jjj75. Ignore the people saying grab the offer like a lottery ticket. I would also add the following:

Living in Doha year round is impossible for your sanity, so you need to budget substantial vacations. The cost of flights are only part of that.

I haven't heard of anyone picking up private insurance individually--usually it's through ones company. AXA, whose UK office you could contact, is active here with direct billing from most hospitals and dentists (at least the Western ones my family goes to).

I think you should budget more the 15k for housing. With bills for six people, I would suggest closer to 18 or 20k a month.

Don't consider anything smaller than a full-sized SUV. Anything smaller is a flirtation with death on these roads. Traffic is horrendous, so you might also have to consider getting two at 3000k per month each (more expensive because the loan repayment terms have to be within the timeframe of your visa).

Get started on finding schools ASAP and count on 25-50k a year. More expensive does not necessarily mean better, but you probably won't have much of a choice. DESS, which is arguably the best primary school in DOHA, is on the cheaper end because it is not-for-profit.

Things generally (with the notable exceptions of domestic labor and petrol) are more expensive here.

Whether or not this is an acceptable deal for you depends on your current situation and how much you want to save. Don't come solely for the money (money can be on the pro list, but it can't be the only pro). People who do usually end up miserable. A good rule of thumb I have heard is that once taxes are considered the salary should be 1.5x whatever you make now plus housing, school fees, flights, medical insurance, car allowance. Otherwise it will be very difficult to save and maintain your current lifestyle.

Good luck!